Techniques for the underwater cleaning of surfaces have for many years relied principally on the use of manual or powered brushes, scrapers, chisels etc.
More recently, in an effort to improve cleaning efficiency, in such applications as the subsea cleaning of welded regions of metal structures prior to safety testing or inspection where high standards of cleaning are demanded, blast cleaning systems employing a high pressure jet of an abrasive slurry have been tried, using water as a carrier for the abrasive. However, the use of such slurries has been found to present many difficulties. For effective cleaning action, the slurry must emerge from the nozzle at a pressure of at least 2000 psig (141 kg/cm.sup.2) above the local hydrostatic (ambient) pressure, more typically from 7000 to 15000 psig (490-1060 kg/cm.sup.2) above the hydrostatic pressure. As well as the need for expensive pumping equipment and components capable of withstanding the very high delivery pressures required, large reactive forces are generated at the nozzle, causing difficulty in orientation and manipulation, and considerable danger to the diver operating the equipment. Furthermore, the equipment is prone to very high degrees of internal abrasion from the high pressure slurry.